Dimetridazole Residues in Pork Tissue. I. Assay by Liquid Chromatography with Electrochemical Detector

Abstract

Abstract A liquid chromatographic (LC) method with electrochemical detection in the reductive mode was developed for the quantitative determination of dimetridazole (DMZ) and its major metabolite (HMMNI) at residue levels in pork tissue. For blood plasma, a sample is precipitated with 2 volumes of acetonitrile and centrifuged, and a diluted aliquot of the supernatant liquid is chromatographed. For muscle, a 10 g sample is extracted 3 times with dichloromethane. After evaporation of the combined extracts, the residue is redissolved in a mixture of hexane and mobile phase (0.3% TEA in 0.6M ammonium acetate pH 5.0 and acetonitrile, 85 + 15) and centrifuged, and an aliquot of the lower phase is chromatographed. Chromatography is accomplished using valve switching with 2 liquid circuits, employing the same mobile phase for both. The sample is deaerated by sparging with helium under slight positive pressure to prevent rediffusion of the oxygen. The sample is first loaded into a deoxygenator and the flow is stopped for complete deoxygenation. The flow is then resumed to transfer the sample into the first, low back-pressure column (ODS, 10 μm, 4.6 x 200 mm). Switching the valve at this point removes the deoxygenator from the circuit and connects the first column to a second one (ODS, 5 μm, 4.6 x 150 mm) in tandem. After the effluent is passed through a second deoxygenator to reduce the residual oxygen in the mobile phase, it is monitored by an electrochemical detector with a screened wall jet cell and a gold mercury electrode, set at —1.2 V. The response is linear, in the ranges studied: 100-1000 pg for DMZ and 200-1000 pg for HMMNI. Mean recoveries for the extraction method are 56 and 70% for DMZ and HMMNI, respectively, using spiked meat and plasma samples at 0.26-2 ppb. Essentially 100% recoveries are obtained with spiked (10 ppb) plasma, using the acetonitrile precipitation